Yes Vyvanse will cause crazy weight loss. My son.was on it for 5 years and initially lost 15 lbs (he was 5) but the school threatend legal action if we didnt continue it. He couldnt eat or gain weight and it seemed like it was stunting his height. Last November I said the heck with the school and took him off it. Since then hes gained 30 lbs and grew almost an inch.